The invention relates to a method of deriving a quality signal which is indicative of the quality of a read signal having constant-length bit cells, which bit cells have a first or second logic value, in which effects represent groups of successive bit cells having a first logic value. In accordance with that method, the read signal is obtained by scanning an information pattern of optically detectable effects, where first read signal portions exceeding a reference value in a first direction correspond to the effects in the read information pattern, and the quality signal is derived on the basis of first read signal portions. The invention further relates to a recording device and a reading device in which the method is used.
A method and reading device of the type described above is known from Research Disclosure, August 1992, No. 34002. That document describes a method of recording what is commonly referred to as EFM-modulated signals used in CD systems. For a reliable recovery of the information represented by the read signal, it is fundamental for the quality of the read signal to satisfy a certain standard. According to the quality standard described in previously mentioned document, the lengths of the first read signal portions in the signal that is read out are, in essence, to adopt discrete values which are equal to an integer number times the bit cell length. The quality signal indicates the extent to which this quality standard is satisfied. According to the known method, first read signal portions corresponding to a small number of bit cells, in this case three, are selected for deriving the quality signal. A signal is derived as the quality signal which is a measure of the jitter of these selected first read signal portions. If this quality signal indicates that the quality standard is not satisfied, a recording strategy is adapted, so that the information pattern and hence also the quality of the read signal is adapted. Recording strategy in this context is meant to be understood as the relationship between the recording beam modulation used for the recording, and the information to be recorded. In the prior-art device, a recording strategy is used according to which an effect having a length that corresponds to 3 bit cells (also termed I3 effect) is recorded by a radiation pulse 2 bit cells in length, the intensity in the last part of the radiation pulse being lower than the intensity at the beginning of the radiation pulse. Effects representing a larger number of bit cells are obtained by extending the radiation pulse, with the intensity in the extended part of the radiation pulse being kept low. If an adaptation of the recording strategy is desired, this adaptation is exclusively effected in the prior-art device by adapting the intensity at the beginning of the radiation pulses.